In orthogonal frequency-division multiplexing (OFDM) based single-frequency networks (SFNs), the same transmit signal is broadcasted simultaneously by various emitting towers on the same carrier frequency. The received signal at the receiver is always the superposition of all transmitted signals. Therefore, the channel impulse response contains all received signal paths, which are individually attenuated due to path loss and delayed due to their physical signal propagation.
Those SFN signals which arrive at the receiver within the timespan of the guard interval contribute to the wanted signal energy. However, those signal parts which arrive outside of the guard interval contribute to inter-symbol-interference, and thus disturb the wanted signal reception.
In a mobile environment, the attenuation of the received SFN signal contributions are continuously changing and fading. Therefore, the time-tracking algorithm at the receiver side is in charge to adjust the timing position for the fast Fourier transform (FFT) window extraction such that the signal-to-interference-plus-noise ratio (SINR) is maximized. For out-of guard channels, this means that the time-tracking algorithm needs to monitor all relevant signal contributions over time, and move the FFT window position accordingly.
For a digital audio broadcasting (DAB) or DAB Plus (DAB+) receiver, the time-tracking algorithm is carried out on an a-priori known preamble symbol, e.g., TFPR symbol. A cross-correlation of the received signal with the perfectly known preamble provides the channel impulse response at the receive antenna. However, the cross-correlation of the preamble symbol has a limited detection range of ±FFT_SIZE/2. Thus, all delay taps which fall within the above detection range can be detected reliably. However, delay taps outside of the detection range become ambiguous and cannot be detected reliably or may even be misinterpreted. In addition, the correlation detection range can also be asymmetric e.g. −FFT_SIZE/4, FFT_SIZE*3/4, which may be difficult to model the power delay profile of the channel impulse response.